I am currently a fourth year hematology/oncology fellow at Duke University. During my T32 supported research fellowship (5T32-HL00572), I focused on describing a novel class of heparin-dependent antibodies originally identified by my mentor's laboratory, protamine/heparin antibodies. This work describing the development of protamine/heparin antibodies after cardiopulmonary bypass surgery was recently published in Blood with a Plenary Paper designation. This year, I successfully competed for a F32 (1F32-AI108118-01), which allowed me to extend my postdoctoral training for one year to generate the preliminary data for this K08 application. In this application, I will establish the biological significance of protamine/heparin antibodies. During my postdoctoral fellowship, I established that protamine/heparin antibodies develop in 29% of patients after a single exposure to protamine and heparin during cardiopulmonary bypass. In my study, as well as others, patients with protamine/heparin antibodies were found to be at risk for adverse clinical outcomes after re-exposure to protamine: arterial thromboembolic events, thrombocytopenia with bleeding, and major adverse cardiovascular events. Although associated with morbidity, the mechanism of protamine/heparin antibody pathogenicity is unknown. In this K08 proposal, I will determine the mechanisms by which protamine/heparin antibodies are pathogenic. In preliminary studies, I demonstrate that protamine/heparin antibodies induce procoagulant monocyte tissue factor activity, proinflammatory monocyte IL-8 expression, and directly bind to extracellular histones. Additionally, I demonstrate the successful development of a murine monoclonal antibody to protamine/heparin, which will facilitate the studies proposed in my research strategy. Based on this strong preliminary data, I will test the hypothesis that protamine/heparin antibodies are pathogenic in settings associated with circulating antigen and inflammation. Specifically, in Aim 1, I will establish that protamine/heparin antibodies exert procoagulant and proinflammatory effects in the presence of protamine, leading to monocyte and neutrophil activation with release of extracellular histones. In Aim 2, I will establish that extracellular histones can serve as an antigen for protamine/heparin antibodies and propagate cellular injury. In Aim 3, I will define the pathogenic features of protamine/heparin antibodies using a murine monoclonal antibody to protamine/heparin that I have recently developed. In completing the proposed aims, I will learn a variety of new in vitro and in vivo techniques, which will be essential to my long-term success as a physician scientist. This K08 award will allow me to develop the skillset necessary for academic success. With this award, I will achieve my immediate goal of understanding the biological significance of protamine/heparin antibodies and also work towards my long-term goal of establishing an independent research career as a physician-scientist in hemostasis and thrombosis. During the period of the award, I will not only master an array of new skills in the laboratory, but I will alo complete didactic coursework on the molecular and cellular basis of the immune response and participate in a variety of workshops to improve my academic skills (manuscript preparation, leadership skills, grant writing, scientific presentation). Duke University is an ideal environment to pursue the research and career development plan outlined in this application. Not only is it a well-established research institution with a multitude of resources available, but my Department and Division are also committed to my overall career development. I am fortunate to have a superb mentor, Gow Arepally, and an advisory committee who are all dedicated to my career development as I transition to an independent scientific career. This K08 award will provide a foundation of protected time as I transition to junior faculty at Duke University.